The present invention relates to a surface acoustic wave device (hereinafter referred to as a SAW device) having a piezoelectric substrate and input/output interdigital transducer formed thereon and more particularly, to a SAW device suitable for use as a band-pass filter in a television receiver set.
The SAW device used as the band-pass filter for the television receiver set includes an input interdigital transducer (hereinafter referred to as an input IDT) and an output interdigital transducer (hereinafter referred to as an output IDT), each of the input and output IDT's having electrode fingers interdigitally arranged on the piezoelectric substrate. The input and output IDT's are spaced apart by a predetermined distance on the piezoelectric substrate and a surface acoustic wave propagating through these IDT's is utilized to obtain a desired frequency characteristic.
A conventional SAW device is constructed as shown in FIG. 7, having a piezoelectric substrate 1, an input IDT 2, an output IDT 3, an earth electrode 4, bus bars 7 and 8 and electrode fingers 9. Reference numeral 5 designates a main lobe and reference numeral 6 designates a side lobe.
In the figure, the input IDT 2 is so arranged as to oppose the output IDT 3 on the piezoelectric substrate 1. Typically, the piezoelectric substrate 1 is made of a material such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO.sub.3). Each of the input and output IDT's 2 and 3 has two bus bars 7 and 8 and electrode fingers 9 provided to each of the bus bars 7 and 8. In the illustrated example, two electrode fingers 9 provided to the bus bar 7 mate with two electrode fingers 9 provided to the bus bar 8 and they are arranged alternately to overlap with each other, thereby forming a split type transducer. In any one of the IDT's 2 and 3, the overlapping aperture of electrode finger is constant to provide a normal type transducer and in the other, the overlapping aperture length of transducer finger is sequentially changed to provide an overlapping aperture weighted transducer.
In the SAW device constructed as above, an electric signal applied to the input IDT 2 is converted into a surface acoustic wave which propagates to the output IDT 3, so that the surface acoustic wave is transferred into an electric signal by means of the output IDT 3. In the SAW filter, a desired filter characteristic is obtained through the operation as above.
Generally, in this type of SAW filter, any one of the IDT's 2 and 3 has the normal type transducer and the other has the aperture weighted transducer as described above and in this example, the input IDT 2 is of the normal type transducer having an overlapping aperture length of W1 and the output IDT 3 is of the overlapping aperture weighted transducer having an overlapping aperture length of W2. The electrode finger overlapping aperture length W2 in the output IDT 3 is maximized at a central portion to provide the main lobe 5 having a maximum overlapping aperture length and is gradually decreased toward the both sides of the central portion to provide the side lobe 6 having a smaller electrode finger overlapping aperture length. Tnterposed between the input and output TDT's is the earth electrode 4 for suppressing a direct feed through (electromagnetic) wave.
Incidentally, in the SAW device, the desirable filter characteristic is so sophisticated that as the band is broadened or the side lobe suppression is increased, the number of electrode fingers 9 constituting the input and output IDT's 2 and 3 must be increased. With the number of the electrode fingers increased, however, the ripple tends to increase within a desired frequency band in the frequency/amplitude characteristic and group delay time characteristic of the SAW device. Examples of known methods of suppressing the ripple are disclosed in Japanese Patent Publication No.57-61211 and Japanese Patent Publication No.3-12485.
An ideal time domain characteristic in the SAW filter has a shape analogous to the electrode finger overlapping aperture length W2 of the aperture weighted transducer 3. Actually, however, when the time domain characteristic of the SAW device is measured, a response waveform as shown in FIG. 8 is obtained.
To explain, a surface acoustic wave reaches the output IDT 3 and then, each time that the surface acoustic wave passes the respective electrode finger 9 of the output IDT 3, an electric signal having an amplitude corresponding to an electrode finger overlapping aperture length W2 of an electrode finger through which the wave passes is obtained. In that case, since the electrode finger overlapping aperture length W2 is maximized in the central portion of the output IDT 3 and is minimized at opposite electrode edges where outermost electrode fingers 9a and 9b are provided, it is expected that the amplitude of the electric signal generated as the surface acoustic wave propagates through the output IDT 3 is initially small, gradually increased, maximized at main lobe A and gradually decreased at side lobe B. Practically, however, a response waveform generated when the surface acoustic wave passes through the outermost electrode finger 9a on the side of input IDT 2 has a very high amplitude as indicated by C' in FIG. 8, as compared to an ideal amplitude indicated by C which is smaller in conformity with an electrode finger overlapping aperture length W2 at the outermost electrode finger 9a. Similarly, a response waveform generated when the surface acoustic wave passes through the outermost electrode finger 9b located at one end opposite to the input IDT 2 has a very high amplitude as indicated by D' in FIG. 8, as compared to an ideal amplitude indicated by D which is smaller in conformity with an electrode finger overlapping aperture length W2 at the outermost electrode finger 9b.
In addition, even after the time that the surface acoustic wave has passed the output IDT 3, a high response waveform as indicated by E in FIG. 8 is obtained.
Because of the generation of the undesired high-amplitude response waveforms C', D' and E, a high ripple is caused within the desired frequency band in the frequency/amplitude characteristic and group delay time characteristic of the SAW device.
These undesired response waveforms generated in the SAW device are of different modes and they are principally classified into the following three kinds on the basis of causes of generation.
(1) Substrate end surface reflection wave: an undesired wave which passes through the input and output IDT's 2 and 3 so as to be reflected at the edge surface of the piezoelectric substrate 1, reaching the output IDT 3.
(2) An electrode end reflection wave: an undesired response wave caused by a surface acoustic wave which is reflected at the electrode end of output IDT 3 at one end opposite to the input IDT 2 so as to again propagate through the output IDT 3 in the reverse direction. This is the undesired response waveform E shown in FIG. 8.
(3) An outermost electrode finger generating wave: an undesired response wave generated at either electrode end of the output IDT 3. This is the undesired response waveform C' or D' shown in FIG. 8.
The substrate end surface reflection wave in the above (1) can be suppressed by forming surface acoustic wave absorbers between an outermost electrode finger 9d of the input IDT 2 and the edge of the piezoelectric substrate 1 and between the outermost electrode finger 9b of the output IDT 3 and the edge of the piezoelectric substrate 1, respectively.
Further, an example of the method for suppression of the electrode end reflection wave in the above (2) is disclosed in the aforementioned Japanese Patent Publication No.57-61211. More specifically, when the input IDT has a aperture weighted transducer and the output IDT has a normal type transducer, a dummy electrode having an electrode aperture length of .lambda./4 (.lambda.: wavelength of the surface acoustic wave) is formed at an outermost electrode finger of the aperture weighted transducer in order to suppress a reflection wave due to reflection of the surface acoustic wave which is generated by the aperture weighted transducer and is then reflected at an edge of aperture weighted transducer at one end opposite to the normal type transducer. The surface acoustic wave reflected at the outermost electrode finger is .lambda./2 or 180.degree. dephased from the surface acoustic wave reflected at the outer end of the dummy electrode and these waves cancel each other so as to be suppressed.